Approaches for business-IT alignment assessment developed in the research community represent an increasing interest for practitioners as they offer an in-depth analysis of the business and IT systems in the organisations. In order to be used by practitioners as a regular tool, these approaches have to be validated. Our experience shows that the perception of validity in academia - "in vitro" - and in the industrial environment - "in vivo" - may differ substantially. In this paper, we discuss the theoretical and empirical (or practical) validity of alignment assessment approaches based on metrics. We propose an empirical validation of a fitness measurement approach for business-IT alignment. First we identify a set of practical validity criteria for this approach and then we generalise our example proposing a set of practical guidelines for operationalisation of approaches based on alignment measurements. Our study reveals a significant gap between our understanding of validity and the perception of our industrial partners about this validity. The contribution of this work is a set of empirical criteria of validity and a set of practical guidelines that can significantly improve the usability - by organisations - of research approaches for business-IT alignment assessment.